Light beam scanning apparatus and image forming apparatus

ABSTRACT

Four beams are caused to scan a scanning surface in the main scanning direction in parallel. The beams expose the target area between two optical sensors, not the surface of a photosensitive drum, to produce correction data to correct shifts in the exposure positions of the four beams in the main scanning direction with an accuracy of less than a small fraction of one pixel. On the basis of the correction data, an actual image formation area is set on the photosensitive drum. This makes it possible to always control the relative exposure scanning position accurately even when the relationship between the main scanning positions of the light beams is unknown.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a light beam scanning apparatus forcausing, for example, laser beams to scan the surface of aphotosensitive drum simultaneously to form an electrostatic latent imageon the photosensitive drum and to an image forming apparatus, such as adigital copying machine or a laser printer, using the light beamscanning apparatus.

[0002] In recent years, various digital copying machines have beendeveloped which form images by, for example, laser-beam scanningexposure and electronic photograph processing.

[0003] To step up the image forming speed, the multi-beam digitalcopying machines have recently been developed. In this type of digitalcopying machine, more than one laser beam is generated and they arecaused to scan in units of lines simultaneously.

[0004] The multi-beam digital copying machine comprises semiconductorlaser oscillators for generating laser beams, a polyhedral rotatingmirror, such as a polygon mirror, and an optical system unit acting as alight-beam scanning apparatus. The polyhedral rotating mirror reflectsthe laser beams emitted from the laser oscillators toward aphotosensitive drum to cause each laser beam to scan the surface of thephotosensitive drum. The optical system unit is composed mainly of acollimator lens and an f-θ lens.

[0005] A method of controlling the exposure position accurately in thedirection in which a laser beam scans (or the main scanning direction)in a digital copying machine of the multi-beam type has been disclosedin, for example, Jpn. Pat. Appln. KOKOKU Publication No. 1-43294, Jpn.Pat. Appln. KOKOKU Publication No. 3-57452, Jpn. Pat. Appln. KOKOKUPublication No. 3-57453, Jpn. UM Appln. KOKOKU Publication No. 5-32824,or Jpn. Pat. Appln. KOKAI Publication No. 56-104572.

[0006] Jpn. Pat. Appln. KOKOKU Publication No. 1-43294 has disclosed amethod of using a light beam sensor to sense the timing with which lightbeams arrive. In this method, the order in which the light beams arriveis unknown. Therefore, the method is not suitable for an optical systemwhere two or more light beam arrive simultaneously.

[0007] Jpn. Pat. Appln. KOKOKU Publication No. 3-57452 has disclosed amethod of providing separate light-receiving sections for sensing lightbeams independently and permitting each light beam to expose thecorresponding light-receiving section and pass through it. On the basisof the signal from each light-receiving section, the light-emittingtiming for printing by each light beam (or recording or image formation)is produced.

[0008] However, for example, when plural recording dot pitches(resolutions), such as 300 dpi, 400 dpi, and 600 dpi, or 16 lines/mm and15.4 lines/mm, are needed, the number of revolutions of the polygonmirror or the frequency of image clock must be changed. In this case, itis difficult to align the print start position of each light beambecause of the following problems: the phase of the output signal fromeach light-receiving section may change with respect to thecorresponding light beam, the timing with which each light beam arrivesat the print start position, and the difference in arriving timingbetween the light beams cannot be divided by one period of image clock.

[0009] The method disclosed in Jpn. Pat. Appln. KOKOKU Publication No.3-57453 has been based on the assumption that the main scanning imageformation area for each light beam is designed to shift in the mainscanning direction. Therefore, the method is not suitable for such anoptical system as is shown in embodiments of the present invention.

[0010] The method disclosed in UM Appln. KOKOKU Publication No. 5-32824is not suitable for such an optical system as is shown in embodiments ofthe present invention because of the same reason as in the method inJpn. Pat. Appln. KOKOKU Publication No. 3-57452.

[0011] The method disclosed in Jpn. Pat. Appln. KOKAI Publication No.56-104572 is to produce a synchronizing signal using one of plural lightbeams and control the light-emitting timing for each light beam on thebasis of the synchronizing signal. The relationship between the scanningpositions of the light beams must be known beforehand. Therefore, themethod is not suitable for such an optical system as is shown inembodiments of the present invention.

BRIEF SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to provide a light beamscanning apparatus and an image forming apparatus which are applicableto an optical system where the order in which light beams are caused toscan in the main scanning direction is unknown (they may be caused toscan simultaneously) and which are capable of constantly controlling theexposure position in the main scanning direction with an accuracy of asmall fraction of a pixel.

[0013] Another object of the present invention is to provide a lightbeam scanning apparatus and an image forming apparatus which areapplicable to more than one resolution.

[0014] In order to achieve the above objects, according to one aspect ofthe present invention, there is provided a light beam scanning apparatuscomprising: a plurality of beam generating means for generating lightbeams; scanning means for optically combining the light beams generatedat the beam generating means, reflecting the combined beams to ascanning surface including the surface of an image retaining element,and causing the light beams to scan the scanning surface; first sensingmeans, provided near the image retaining element for sensing the firstone of the light beams caused by the scanning means to scan; clockgenerating means for generating a pixel clock to be used in the beamgenerating means for each of the light beams, in response to a sensesignal indicating that the first beam has exposed the first sensingmeans; second sensing means, provided on the more downstream side in themain scanning direction than the first sensing means and, for sensingthe light beams; first control means for giving control data to theclock generating means so that the light beams may expose a target areabetween the first and second sensing means; and image formation areasetting means for determining a pixel clock area corresponding to atarget image formation area on the image retaining element on the basisof the control data from the control means and setting the pixel clockarea in the clock generating means.

[0015] Accordingly, four beams, which are parallel with each other inthe sub-scanning direction, are caused to scan a scanning surface in themain scanning direction. The beams expose the target area between twooptical sensing means, not the surface of the image retaining element,to produce correction data to correct shifts in the exposure positionsof the four beams in the main scanning direction with an accuracy ofless than a small fraction of one pixel. On the basis of the correctiondata, an image formation area is set on the image retaining element.

[0016] The clock generating means includes: clock means for generating aclock signal a specific time after the first beam exposed the firstsensing means; delay means which delays the clock signal generated atthe clock means, selects the delay of the clock in a range of one clockor less for each beam, and provides a delayed clock signal as a pixelclock to be used to generate a beam; and clock setting means forsetting, for the respective beams, exposure pixel clock areas used bythe beam generating means in the pixel clocks given by the delay meansand providing exposure pixel clocks.

[0017] The delay means includes a delay line and delay clock selectors,the delay line having taps, and each of the delay clock selectorsdesigned to select and output a delayed clock generated at one tap,which is to be used to generate one beam.

[0018] Specifically, to align the image formation positions of the fourbeams accurately in the main scanning direction, the amount of delay isgiven to the clock signal with an accuracy of one-tenth of a clock foreach of the beam generating means independently.

[0019] The target area between the first and second sensing means is anarea overlapping with the second sensing means, the light beam scanningapparatus further comprising: a counter for counting the number of timesthe beams expose the second sensing means; second control means fordriving one of the beam generating means using the exposure pixel clockobtained from the clock setting means controlled on the basis of thecontrol data and causing the scanning means to scan the scanning surfacea specific number of times; comparison means for reading a value in thecounter after the second control means has scanned the specific numberof times and comparing the value with a predetermined number; means forselecting the next tap whose amount of delay is greater than that of theselected tap when the result of the comparison at the comparison meanshas shown that the value in the counter is smaller than thepredetermined number; and amount-of-delay setting means, when the resultof the comparison at the comparison means has shown that the value inthe counter is equal to the predetermined number, for setting a delayselected at that time as the amount of delay for the one of the beamgenerating means.

[0020] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0021] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate presently preferredembodiments of the invention, and together with the general descriptiongiven above and the detailed description of the preferred embodimentsgiven below, serve to explain the principles of the invention.

[0022]FIG. 1 schematically shows the configuration of a digital copyingmachine according to an embodiment of the present invention;

[0023]FIG. 2 shows the configuration of an optical system unit and thelocation of a photosensitive drum;

[0024]FIG. 3 schematically shows the configuration of a light beamsensing unit;

[0025]FIG. 4 is a block diagram of a control system for mainlycontrolling the optical system;

[0026]FIG. 5 shows the positional relationship between the light beamsensing unit and the photosensitive drum and the exposure area (printingarea) of each light beam by a sample timer as well as the positionalrelationship between light-emitting areas by image data together with atime chart;

[0027]FIG. 6 is a block diagram of a configuration for setting aprinting area (exposure area) in as small units as one clock or less;

[0028]FIG. 7 is a diagram to help explain the operation of a clocksynchronizing circuit;

[0029]FIG. 8 is a diagram to help explain the principle of a method ofacquiring information on the main scanning beam position for each lightbeam;

[0030]FIG. 9 is a diagram to help explain a method of sensing therelative positional relationship between each light beam and each outputof the light beam sensing unit;

[0031]FIG. 10 is a diagram to help explain the operation of themain-scanning light-beam position sensing circuit;

[0032]FIG. 11 is a diagram to help explain the operation of themain-scanning light-beam position sensing circuit;

[0033]FIG. 12 is a diagram to help explain the operation of themain-scanning light-beam position sensing circuit;

[0034]FIG. 13 schematically shows exposure areas when the light beampower differs;

[0035]FIG. 14 is a block diagram to help explain passing positioncontrol of light beams and an offset sensing and correcting process;

[0036]FIG. 15 is a flowchart to help explain a general operation at thetime of turning on the power supply of the printer section;

[0037]FIG. 16 schematically shows the configuration of the light beamsensing unit complying with two resolutions;

[0038]FIG. 17 is a flowchart to help explain a first example of alight-beam power control routine;

[0039]FIG. 18 is a flowchart to help explain the first example of thelight-beam power control routine;

[0040]FIG. 19 is a flowchart to help explain the main-scanninglight-beam position control routine;

[0041]FIG. 20 is a flowchart to help explain the routine for acquiringinformation on the main-scanning light-beam position of a light beam;

[0042]FIG. 21 is a flowchart to help explain the routine for acquiringinformation on the main-scanning light-beam position of the light beam;and

[0043]FIG. 22 is a flowchart to help explain the routine for acquiringinformation on the main-scanning light-beam position of the light beam.

DETAILED DESCRIPTION OF THE INVENTION

[0044] Hereinafter, referring to the accompanying drawings, anembodiment of the present invention will be explained.

[0045]FIG. 1 shows the configuration of a digital copying machine, whichis an image forming apparatus to which a light beam scanning apparatusaccording to an embodiment of the present invention is applied. Thedigital copying machine includes a scanner unit 1 acting as imagereading means and a printer unit 2 acting as image forming means. Thescanner unit 1 is composed of a first carriage 3 and a second carriage 4which are movable in the direction of the arrow, an image formation lens5, and a photoelectric conversion element 6.

[0046] In FIG. 1, a document O is placed on an document table 7 made oftransparent glass, with the printed side downward. The placementreference of the document is at the front right on the lateral sidealong the width of the document table 7. A document holding cover 8 thatcan be opened and closed presses the document O against the documenttable 7.

[0047] A light source 9 illuminates the document 9. The reflected lightfrom the document passes through mirrors 10, 11, 12 and the imageformation lens 5 and is gathered on the light-receiving surface of thephotoelectric conversion element 6. The first carriage 3 on which thelight source 9 and mirror 10 are mounted and the second carriage 4 onwhich the mirrors 11, 12 are mounted move at a relative speed in theratio of 2:1 to make the optical path length constant. A carriagedriving motor (not shown) moves the first carriage 3 and second carriage4 from right to left in synchronization with a read timing signal.

[0048] In this way, the image of the document O on the document table 7is read line by line by the scanner unit 1. The output of the scannerunit 1 is converted by an image processing unit (not shown) into an8-bit digital signal indicating the gradation of image.

[0049] The printer unit 2 is composed of an optical system unit 13 andan image forming unit 14 using an electronic photographic system capableof forming an image on a sheet of paper P serving as a medium on whichan image is to be formed. Specifically, the image signal read from thedocument O by the scanner unit 1 is processed at the image processingunit (not shown) and then converted into laser beams (hereinafter,referred to as light beams) from semiconductor laser oscillators. Theoptical system of the embodiment uses a multi-beam optical system usingmore than one semiconductor laser oscillator.

[0050] The configuration of the optical system unit 13 will be explainedin detail later. The semiconductor laser oscillators provided in theunit emit light according to the laser modulation signal outputted fromthe image processing unit (not shown). The light beams from thesemiconductor laser oscillators are reflected by a polygon mirror andoutputted to the outside of the unit in the form of scanning light.

[0051] The light beams from the optical system unit 13 form a spot withthe necessary resolution at point X, the exposure position on aphotosensitive drum 15 serving as an image retaining member. The spot iscaused to scan the photosensitive drum 15 for exposure. This makes anelectrostatic latent image corresponding to the image signal appear onthe photosensitive drum 15.

[0052] Around the photosensitive drum 15, there are provided anelectrifying charger 16 for electrifying the surface of the drum, adeveloping unit 17, a transfer charger 18, a peeling charger 19, and acleaner 20. The photosensitive drum 15 is rotated by a driving motor(not shown) at a specific circumferential speed. It is electrified bythe electrifying charger 16 facing the surface of the drum. More thanone light beam (or scanning light beam) forms a spot at point x, theexposure position on the electrified photosensitive drum 15.

[0053] The electrostatic latent image formed on the photosensitive drum15 is developed with the toner (or developer) from the developing unit17. The toner image formed on the photosensitive drum 15 is transferredat the transfer position onto the sheet P fed with a suitable timing bythe paper feed system.

[0054] In the paper feed system, sheets of paper P in a paper feedcassette 21 are taken out one by one by a supply roller 22 and aseparation roller 23. The sheet P is sent to a resist roller 24, whichcarries the sheet to the transfer position with a specific timing. Onthe downstream side of the transfer charger 18, there are provided asheet transport mechanism 25, a fixing unit 26, and delivery rollers 27for discharging the sheet P on which an image has been formed. With thisarrangement, the fixing unit 26 fixes the toner image on the sheet P onwhich the toner image has been transferred. Thereafter, the sheet isdischarged via the delivery rollers 27 into a delivered sheet tray 28outside.

[0055] After the image has been transferred from the photosensitive drum15 to the sheet P, the remaining toner on the surface of the drum isremoved by the cleaner 20, which returns the drum to the initial state.In this state, the drum stands by to form the next image.

[0056] The repetition of the aforementioned processes causes theoperation of forming images to be performed continuously.

[0057] As described above, the document O on the document table 7 isread at the scanner unit 1. The read data is subjected to a series ofprocesses at the printer unit 2 and then recorded on the sheet P in theform of a toner image.

[0058] The following is explanation of the optical system unit 13.

[0059]FIG. 2 shows the configuration of the optical system unit 13 andthe location of the photosensitive drum 15. The optical system unit 13includes, for example, semiconductor laser oscillators 31 a, 31 b, 31 c,31 d, serving as four light-beam generating means. The semiconductorlaser oscillators 31 a, 31 b, 31 c, 31 d scan line by linesimultaneously, enabling an image to be formed at high speed withoutincreasing the number of revolutions of the polygon mirror.

[0060] Specifically, the laser oscillator 31 a is driven by a laserdriver 32 a. The outputted light beam passes through a collimator lens(not shown) and strikes a galvanomirror 33 a serving as optical pathchanging means. The light beam reflected from the galvanomirror 33 apasses through a half mirror 34 a and a half mirror 34 b and strikes apolygon mirror 35 acting as a multiplanar rotation mirror.

[0061] The polygon mirror 35 is rotated at a constant speed by a polygonmotor 36 driven by a polygon motor driver 37. This causes the reflectedlight from the polygon mirror 35 to scan in a constant direction at anangular velocity determined by the number of revolutions of the polygonmotor 36. The light beam moved to and fro by the polygon mirror 35passes through an f-θ lens (not shown). The f-θ characteristic of thelens enables the light beam to scan the light-receiving surface of alight beam sensing unit 38 and the surface of the photosensitive drum 15at a constant speed. The beam sensing unit 38 serves as light-beampassage sensing means and light-beam position sensing means.

[0062] The laser oscillator 31 b is driven by a laser driver 32 b. Theoutputted light beam passes through a collimator lens (not shown) and isreflected by a galvanomirror 33 b and then by the half mirror 34 a. Thereflected light from the half mirror 34 a passes through the half mirror34 b and strikes the polygon mirror 35. The optical path through whichthe beam travels after the polygon mirror 35 is the same as that for thelaser oscillator 31 a. Namely, the light beam passes through the f-θlens (not shown) and scans the light-receiving surface of the light beamsensing unit 38 and the surface of the photosensitive drum 15 at aconstant speed.

[0063] The laser oscillator 31 c is driven by a laser driver 32 c. Theoutputted light beam passes through a collimator lens (not shown) and isreflected by a galvanomirror 33 c. The reflected light passes through ahalf mirror 34 c, is reflected by the half mirror 34 b, and strikes thepolygon mirror 35. The optical path through which the beam travels afterthe polygon mirror 35 is the same as that for the laser oscillator 31 aor 31 b. Namely, the light beam passes through the f-θ lens (not shown)and scans the light-receiving surface of the light beam sensing unit 38and the surface of the photosensitive drum 15 at a constant speed.

[0064] The laser oscillator 31 d is driven by a laser driver 32 d. Theoutputted light beam passes through a collimator lens (not shown) and isreflected by a galvanomirror 33 d. The reflected light is furtherreflected by the half mirror 34 c and then by the half mirror 34 b, andstrikes the polygon mirror 35. The optical path through which the beamtravels after the polygon mirror 35 is the same as that for the laseroscillator 31 a, 31 b, or 31 c. Namely, the light beam passes throughthe f-θ lens (not shown) and scans the light-receiving surface of thelight beam sensing unit 38 and the surface of the photosensitive drum 15at a constant speed.

[0065] Each of the laser drivers 32 a to 32 d includes an automaticpower control (APC) circuit. The APC circuits cause the laseroscillators 31a to 31d to emit light constantly at the light-emittingpower level set by a main control unit (CPU) 51, which will be explainedlater.

[0066] The light beams from the separate laser oscillators 31 a, 31 b,31 c, 31 d are combined at the half mirrors 34 a, 34 b, 34 c to formfour light beams, which travel toward the polygon mirror 35.

[0067] This enables the four light beams to scan the photosensitive drum15 simultaneously. As a result, if the number of revolutions of thepolygon mirror 35 is the same, use of the four light beams will enablean image to be recorded at a speed four times as fast as that achievedby use of a single light beam in the prior art.

[0068] The galvanomirrors 33 a, 33 b, 33 c, 33 d are for adjusting (orcontrolling) the positional relationship between the light beams in thesub-scanning direction (or in the direction perpendicular to the mainscanning direction). Galvanomirror driving circuits 39 a, 39 b, 39 c, 39d are connected to the galvanomirrors 33 a, 33 b, 33 c, 33 d,respectively.

[0069] The light beam sensing unit 38 is for sensing the passingposition, passing timing, and power of each of the four light beams. Itis provided near one end of the photosensitive drum 15 in such a mannerthat the light-receiving surface of the unit 38 becomes flush with thesurface of the photosensitive drum 15. Control of the galvanomirrors 33a, 33 b, 33 c, 33 d corresponding to the respective light beams (orimage formation position control in the sub-scanning direction), controlof the light-emitting power (or intensity) of the laser oscillators 31a, 31 b, 31 c, 31 d, and control of the light emitting timing (or imageformation position control in the main scanning direction) are performedon the basis of the sense signal from the light beam sensing unit 38. Alight-beam sensor output processing circuit 40 is connected to the lightbeam sensing unit 38 to generate the signals for performing the abovecontrols.

[0070] The following is explanation of the light beam sensing unit 38.

[0071]FIG. 3 pictorially shows how the structure of the light beamsensing unit 38 is related to the direction in which the light beamsscan. The four light beams a to d from the four semiconductor laseroscillators 31 a, 31 b, 31 c, 31 d scan as the polygon mirror 35 rotatesin such a manner that they traverse over the light beam sensing unit 38from left to right.

[0072] The light beam sensing unit 38 comprises two oblong sensorpatterns S1, S2 acting as a first light sensing section, seven sensorpatterns SA, SB, SC, SD, SE, SF, SG sandwiched between the two sensorpatterns S1, S2 and serving as a second and a third light sensingsection, and a holding substrate 38 a serving as a holding member forintegrally holding the sensor patterns S1, S2, SA, SB, SC, SD, SE, SF,SG. The sensor patterns S1, S2, SA to SG are composed of, for example,photodiodes.

[0073] The sensor pattern S1 is a pattern that senses the passing of alight beam and generates a reset signal (or an integration start signal)for an integrator explained later. The sensor pattern S2 is a patternthat senses the passing of a light beam and generates a conversion startsignal for an A/D converter explained later. The sensor patterns S1, S2function as reference patters in various control actions in the mainscanning direction. Each of the sensor patterns SA to SG is a patternthat senses the passing of a light beam.

[0074] As shown in FIG. 3, the sensor patterns S1, S2 are formed oblongin the direction perpendicular to the direction in which the light beamsscan so that the light beams a to d deflected by the polygon mirror 35never fail to traverse them, regardless of the positions of thegalvanomirrors 33 a to 33 d. For example, in the embodiment, the widthW1 of the pattern S1 and the width W3 of the pattern S2 in the mainscanning direction are 200 μm, whereas the length L1 of them in thedirection perpendicular to the direction in which the light beams scanis 2000 μm.

[0075] The sensor patterns SA to SG are arranged in such a manner thatthey are stacked one on top of another in the direction perpendicular tothe direction of light beam scanning between the sensor patterns S1 andS2 as shown in FIG. 3. The length along which they are arranged is L1,the same length as that of the sensor patterns S1, S2. The width W2 ofeach of the sensor patterns SA to SG in the direction of light beamscanning is, for example, 600 μm.

[0076] To sense the power of the light beam on the photosensitive drum15, the passing position of the light beam is controlled so that thelight beam may pass over sensor pattern SA or SG as shown by abroken-line arrow Pa or Pb in FIG. 3. Then, the output of sensor patternSA or SG is taken in.

[0077] The following is explanation of the control system.

[0078]FIG. 4 shows the control system for mainly controlling themulti-beam optical system. Numeral 51 indicates a main control unit 51which is composed of, for example, a CPU, and supervises the overallcontrol. Connected to the main control unit 51 are a memory 52, acontrol panel 53, an external communication interface (I/F) 54, thelaser drivers 32 a, 32 b, 32 c, 32 d, the polygon mirror motor driver37, the galvanomirror driving circuits 39 a, 39 b, 39 c, 39 d, the lightbeam sensor output processing circuit 40, a synchronizing circuit 55,and an image data interface (I/F) 56.

[0079] The image data I/F 56 is connected to the synchronizing circuit55. An image processing unit 57 and a page memory 58 are connected tothe image data I/F 56. The scanner unit 1 is connected to the imageprocessing unit 57. An external interface (I/F) 59 is connected to thepage memory 58.

[0080] The flow of image data in forming an image will be explainedbriefly.

[0081] As explained earlier, in a copying operation, the image on thedocument O set on the document table 7 is read by the scanner unit 1 andthe read signal is sent to the image processing unit 57. The imageprocessing unit 57 subjects the image signal from the scanner unit 1 toknown shading correction, various filtering processes, gray levelprocessing, and gamma correction.

[0082] The image data from the image processing unit 57 is sent to theimage data I/F 56. The image data I/F 56 distributes the image data tothe four laser drivers 32 a, 32 b, 32 c, and 32 d.

[0083] The synchronizing circuit 55 generates a clock synchronizing withthe timing with which each light beam passes over the light beam sensingunit 38 and sends the image data from the image data I/F 56 to the laserdrivers 32 a, 32 b, 32 c, and 32 d as a laser modulation signal insynchronization with the clock.

[0084] Transferring the image data in synchronization with the scanningof each light beam enables an image to be formed (in the properposition) synchronously in the main scanning direction.

[0085] The synchronous circuit 55 includes a sample timer and a drumexposure inhibit timer. The sample timer forces the laser oscillators 31a, 32 b, 31 c, 31 d to emit light in non-image areas and controls thepower of each light beam. The drum exposure inhibit timer prevents theforced light emission by the main control unit 51 from exposing thephotosensitive drum 15 in performing control of the light-beam passing(or scanning) position or control of the powers of the individual lightbeams, which will be explained later.

[0086] The control panel 53 is a man-machine interface for starting acopying operation or setting the number of sheets of paper.

[0087] The digital copying machine of the embodiment can not only makecopies but also form images from the image data externally supplied viathe external I/F 59 connected to the page memory 58. The image datasupplied from the external I/F 59 is temporarily stored in the pagememory 58 and then sent to the synchronizing circuit 55 via the imagedata I/F 56.

[0088] When the digital copying machine is externally controlled via anetwork, the external communication I/F 54 serves as the control panel53.

[0089] The galvanomirror driving circuits 39 a, 39 b, 39 c, and 39 d arecircuits for driving the galvanomirrors 33 a, 33 b, 33 d, and 33 daccording to the specified values from the main control unit 51.Therefore, the main control unit 51 can control the angles of thegalvanomirrors 33 a, 33 b, 33 d, and 33 d freely via the galvanomirrordriving circuits 39 a, 39 b, 39 c, and 39 d.

[0090] The polygon motor driver 37 is a driver for driving the polygonmotor 36 for rotating the polygon mirror 35 that defects the four lightbeams. The main control unit 51 instructs the polygon motor driver 37 tostart and stop rotation or change the number of revolutions. Changingthe number of revolutions is effected when the recording pitch (orresolution) is changed.

[0091] The laser drivers 32 a, 32 b, 32 c, 32 d have the function of notonly emitting laser light according to the laser modulation signal insynchronization with the scanning of the line beam from thesynchronizing circuit 55 but also forcing the laser oscillators 31 a, 31b, 31 c, 31 d to emit light according to the forced light-emittingsignal from the main control unit 51, regardless of the image data.

[0092] The function is used when the laser oscillators 31 a, 31 b, 31 c,31 d are forced to emit light in performing control of the light-beampassing (or scanning) position or control of the powers of theindividual light beams. As explained earlier, however, the drum exposureinhibit timer in the synchronizing circuit 55 prevents the forced lightemission over the photosensitive drum 15.

[0093] The main control unit 51 sets the power produced by each of thelaser oscillators 31 a, 31 b, 31 c, 31 d in the respective laser drivers32 a, 32 b, 32 c, 32 d. The setting of the light-emitting power ischanged according to changes in the processing conditions or the sensingof the passing position of the light beam.

[0094] The memory 52 is for storing the necessary data for control. Itstores, for example, the controlled variables for the galvanomirrors 33a, 33 b, 33 c, 33 d, the characteristic of a circuit for sensing thepassing position of a light beam (or the offset value of an amplifier),and the printing area corresponding to each light beam. This enables theoptical system unit 13 to be brought into the image formation modeimmediately after the power supply has been turned on.

[0095] Hereinafter, light-beam position control (or printing areasetting)-in the main scanning direction will be explained in detail.

[0096]FIG. 5 shows the positional relationship between the sensorpatterns S1, S2 of the light beam sensing unit 38 and the photosensitivedrum 15 as well as the exposure area (printing area) of the light beamsa to d by a sample timer explained later and the light-emitting area onthe basis of the image data. FIG. 5 also shows the positionalrelationship with the output of the drum exposure inhibit timer togetherwith a time chart. The circuit configuration to realize the operationwill be explained in detail by reference to FIG. 6.

[0097] As shown in FIG. 5, the output of sensor pattern S1 in the lightbeam sensing unit 38 resets the sample timer (at time t0), which thenstarts to count the clock (not shown) from 0. When the count of thesample timer has reached a specific value, the output of the sampletimer is high as shown at time t2, which forces the four laseroscillators 31 a to 31 d to emit light. The value set in the sampletimer is normally such a value as causes the light beams a to d to beemitted before the next polygon mirror surface deflects them after theyhave passed the photosensitive drum 15, as shown in FIG. 5.

[0098] When the next polygon mirror surface has started to deflect thelight beams a to d and the first light beam has reached sensor patternS1 (at time t3), the sample timer is reset and the above-describedoperation is repeated. Specifically, the laser oscillators 31 a to 31 dare forced to emit light line by line in the area irrelevant to imageformation at regular intervals of time (from time t2 to t3). During theforced light emission, automatic power control (APC) to keep thelight-emitting power of a laser beam at a specific value is performedfor each of the laser oscillators 31 a to 31 d.

[0099] The following is an explanation of the drum exposure inhibittimer. Forced light emission includes not only light emission by theoutput of the sample timer but also the aforementioned light emissionthat the main control unit 51 directly causes the laser drivers 32 a to32 d to effect. In the forced light emission, the main control unit 51causes any one of the laser oscillators 31 a to 31 d to emit light andthe light to scan over the light beam sensing unit 38. In the scanning,light-beam passing (scanning) position control and light-beam powercontrol of the beams are performed.

[0100] When the laser oscillators 31 a to 31 d are forced to emit lightconsecutively, the photosensitive drum 15 is exposed continuously, whichleads to the following problems.

[0101] When the photosensitive drum 15 is at a stop, the beams expose aspecific portion of the photosensitive drum 15 intensively, which canlead to a local degradation of the photosensitive drum 15. When thephotosensitive drum 15 is rotating, a lot of toner may adhere (or areconsumed) or the carrier may adhere to the drum.

[0102] The drum exposure inhibit timber avoids those problems. When thetimer is in operation, it prevents the main control unit 51 fromeffecting forced light emission in the area (from time t1 to t2)including the photosensitive drum area, as shown in the time chart ofFIG. 5. Specifically, the light beams pass over the light beam sensingunit 38 on the basis of the output of sensor pattern S1 of the lightbeam sensing unit 38. The forced emission is inhibited (the output ofthe drum exposure inhibit timer: high) before the light beams approachthe photosensitive drum 15 (from time t2: T_(OFF1) has elapsed since theoutput of S1). At the time when the light beams have finished passingover the photosensitive drum 15 (at time t2: T_(OFF2) has elapsed sincethe output of S1), the inhibition of forced light emission is canceled(the output of the drum exposure inhibit timer: low).

[0103] Light emission on the basis of the image data (including the testimage data) is normally effected on the printing area of thephotosensitive drum 15 as shown in FIG. 5. In the case of aconfiguration where light beams are combined by a half mirror and causedto scan, the positional relationship between light beams in the mainscanning direction is not constant. FIG. 5 shows a case where light beama is at the head, followed by light beams b, c, and d in that order. Asshown in FIG. 5, with light beam a as a reference, light beam b isdelayed ΔTab, light beam c is delayed. ΔTac, and light beam d is delayedΔTad.

[0104] To make the light beams a to d having such a positional (phase)relationship coincide exactly with the exposure area, it is necessary toshift the light-emitting timing based on the image data by ΔTab forlight beam b, ΔTac for light beam c, and ATad for light beam d withlight beam a as a reference as shown in FIG. 5.

[0105] In setting the exposure area, adjustments are generally made inunits of one clock (or in units of one pixel) on the basis of areference clock. In the optical system of the present embodiment, thereis no guarantee the positions of the light beams are shifted in units ofone clock. Therefore, closer adjustments are needed.

[0106]FIG. 6 shows a configuration for setting the printing area(exposure area) in units of less than one clock and a configuration forpreventing the forced light emission from exposing the drum. Theseconfigurations correspond to the synchronizing circuit 55, light beamsensor output processing circuit 40, and light beam sensing unit 38.

[0107] In FIG. 6, a main-scanning light-beam position sensing circuit 40a is composed of a first counter 111, a second counter 112, and a latchcircuit 113.

[0108] The synchronizing circuit 55 comprises four crystal oscillators114 a to 114 d, a selector 115 for selecting one of the crystaloscillators 114 a to 114 d, a clock synchronizing circuit 116, a delayline 117, four delay clock selectors 118 a to 118 d, four image transferclock generating sections (printing area setting sections) 119 a to 119d, a sample timer 120, an OR gate circuit 121, and a drum exposureinhibit timer 122. One of the crystal oscillators 114 a to 114 d isselected by the selector 115 according to image resolution.

[0109] More detailed explanation will be given by reference to FIG. 6.

[0110] First, the configuration for preventing the forced light emissionfrom exposing the drum will be described. As shown in FIG. 6, the maincontrol unit 51 is capable of sensing a forced light-emission signal toeach of the laser drivers 32 a to 32 d and forcing the laser oscillators31 a to 31 d to emit light.

[0111] The laser drivers 32 a to 32 d are designed not to receive theforced light-emission signal from the main control unit 51 during thetime when the drum exposure inhibit timer 122 continues outputting thedrum exposure inhibit signal (from time t1 to time t2 in FIG. 5). Duringthe period, the laser oscillators 31 a to 31 d will not emit light evenwhen the main control unit 51 has outputted the forced light-emissionsignal.

[0112] The operation of the drum exposure inhibit timer 122 iscontrolled by an enable/disable signal from the main control unit 51. Incontrolling the light-beam passing position and the power, the maincontrol unit 51 outputs the enable signal to the drum exposure inhibittimer 122, which then starts to operate. Specifically, a drum exposureinhibit signal is outputted to the laser drivers 32 a to 32 d during theperiod from time t1 to time t2 in FIG. 5. Therefore, even when the maincontrol unit 51 outputs the forced light-emission signal to the laserdrivers 32 a to 32 d, the laser oscillators 31 a to 31 d are preventedfrom emitting light at least during the period. Accordingly, the valuescorresponding to time T_(OFF1) and T_(OFF2) in FIG. 5 are set in thedrum exposure inhibit timer.

[0113] On the other hand, when a normal image is formed or when thelaser drivers 32 a to 32 d operate according to the input image data,the main control unit 51 outputs the disable signal to the drum exposureinhibit timer 122. As a result, the drum exposure inhibit timer 122 isprevented from outputting the drum exposure inhibit signal, which allowsthe light beam to be projected onto the printing area on the drumaccording to the image data.

[0114] Therefore, in controlling the beam position and the power, themain control unit 51 outputs the enable signal to the drum exposureinhibit timer 122 and then the forced light-emission signal to the laserdrivers 32 a to 32 d. As a result, the main control unit 51 can exposethe light beam sensing unit 38 using any one of the light beams withouttaking into account the movement of the light beam (or without exposingthe photosensitive drum 15).

[0115] Next, the configuration for setting the printing area (exposurearea) in units of less than one clock will be described. As shown inFIG. 5, the sensor pattern S1 of the light beam sensing unit 38 isexposed by the first of the light beams a, b, c, and d forced by thesample timer 120 to emit light and changes the signal level from low tohigh. The high signal is inputted to the sample timer 120, which cancelsall of the forced emission by the laser oscillators 31 a to 31 d.

[0116] Consequently, the light beams a, b, c, and d disappear andtherefore the high output of the sensor pattern S1 also disappears.

[0117] The output of the sensor pattern S1 is also inputted to the clocksynchronizing circuit 116 in the synchronizing circuit 55. The clocksynchronizing circuit 116 outputs a synchronized clock. The synchronizedclock is a clock that rises ΔT_(SYNC) after the trailing edge of theoutput of the sensor pattern S1 as shown in FIG. 7. The synchronizedclock is also synchronized with the output of the sensor pattern S1 andhas the same frequency as that of the output clock of the crystaloscillators.

[0118] Then, the synchronized clock is inputted to the delay line 117.The delay line 117 has the function of delaying the input signal for aspecific time. The delay line 117 shown in the figure has ten taps foroutput. Specifically, delayed clock Dl outputted from a first-stage tapis delayed Δtd from the inputted synchronized clock and delayed clock D2outputted from a second-stage tap is delayed another Δtd from theinputted synchronized clock. The delay of Δtd is actually severalnanoseconds.

[0119] Then, delayed clock D10 outputted from the last-stage(tenth-stage) tap is delayed 10·Δtd from the inputted synchronizedclock. In the embodiment, one-tenth of a period of the synchronizedclock is almost equal to Δtd. That is, the delayed clock D10 is almostequal to the inputted synchronized clock, or to a clock shifted oneclock.

[0120] In the embodiment, the amount of delay in the delay line 117 isset at one-tenth of a clock. When the printing area has to be set moreaccurately, the amount of delay per tap should be made smaller and thenumber of taps be increased.

[0121] The outputs of the delay line 117, or the delayed clocks D1 toD10, are inputted to the delayed clock selectors 118 a to 118 dcorresponding to the light beams a to d. The delayed clock selectors 118a to 118 d has the function of selecting clocks and outputting them tothe image transfer clock generating sections (printing area settingsections) 119 a to 119 d at the next stage on the basis of the delayedclock select signal outputted from the main control unit 51. In otherwords, the main control section 51 can freely select one from thedelayed clocks D1 to D10 for each of the light beams a to d to set thesetting area.

[0122] Here, the image transfer clock generating sections (printing areasetting sections) 119 a to 119 d will be explained. Using the printingarea setting signal, the main control unit 51 sets a printing area foreach of the light beams a to d in units of one clock (in units of onepixel). Namely, it can set the output timing of the image transfer clockand the number of outputs. In foming a normal image, it sets thosefactors so that the image formation area for the light beams a to d maybe a target image formation area on the photosensitive drum 15. Thetarget image formation area varies with the sheet size to be used or thefiling margin.

[0123] Now, the image transfer clock (printing area signal) thusobtained is sent to the image data I/F 56, which outputs the image data(laser modulation signals) corresponding to the light beams a to d insynchronism with the image transfer clock (printing area signal). Thelaser drivers 32 a to 32 d modulate the laser oscillators 31 a to 31 don the basis of the image data (laser modulation signal). The clocksignal outputted from the image transfer clock generating section duringthe main scanning position adjustment is used as an exposure pixelclock. The corresponding laser oscillator 31 is forced to emit lightduring the clock generation period.

[0124] In this way, the main control unit 51 can set a printing area inthe image transfer clock generating sections (printing area settingsections) 119 a to 119 d in units of one clock (in units of one pixel)on the basis of the printing area setting signal. It can also set aprinting area in units of one-tenth of a clock (in units of one-tenth ofa pixel) for each of the light beams a to d by the delayed clock selectsignals to the delayed clock selectors 118 a to 118 d.

[0125] Next, the principle of acquiring main-scanning beam positioninformation about the light beams a to d for the main control unit 51 toset a printing area in units of one clock (in units of one pixel) or inunits of one-tenth of a clock (in units of one-tenth of a pixel) will beexplained by reference to FIG. 8.

[0126]FIG. 8 shows a case where a much smaller exposure area than informing a normal image has been set in the image transfer clockgenerating section (printing area setting section) 119 a. The maincontrol unit 51 first selects delayed clock D1 for, for example, lightbeam a, sets a printing area to 1-5, and gives the image data I/F 56 atest printing instruction to paint black all over the area (or projectlaser light in the printing area). As shown in the figure, when thefirst beam (in this case, beam a) has exposed sensor pattern S1, all ofthe four beams a to d are turned off. Thereafter, only light beam aexposes the section of printing area 1-5.

[0127] As described above, when a small value is set for the printingarea, the light beam does not reach the area of the photosensitive drum15 and exposes the light beam sensing unit 38. In this state, monitoringthe output of sensor pattern S2 on the downstream side of sensor patternS1, the main control unit 51 determines what size the printing areashould be set at in order to allow sensor pattern S2 to response. Asseen from the example of FIG. 8, setting the printing area to 6-10allows sensor pattern S2 to start to respond.

[0128] In this way, the main control unit 51 can sense the relativepositional relationship of light beam a with the output of sensorpattern S1 in units of one clock (in units of one pixel).

[0129] Next, a method of sensing the relative positional relationship oflight beam a with the output of sensor pattern S1 in units of less thanone clock (in units of less than one pixel) will be explained byreference to FIG. 9. As explained in FIG. 8, when delayed clock D1 hasbeen selected for light beam a, setting the printing area to 6-10 allowssensor pattern 52 to respond. Thus, the main control unit 51 reduces theset value of the printing area to 5-9 and changes the selection of thedelayed clock.

[0130] As shown in FIG. 9, as the selection of the delayed clock ischanged in this order: D1→D2→D3, the printing area moves right in unitsof one-tenth of a clock (in units of one-tenth of a pixel). In thepresent embodiment, when delayed clock D5 has been selected, sensorpattern S2 starts to respond.

[0131] Therefore, in a case where the main control unit 51 has set theprinting area (exposure area) to five pixels on the basis of the outputof sensor pattern S1 for light beam a, has set the area to 5-9 and hasselected delayed clock D5, it senses that the right end of the printingarea has exposed sensor pattern. In other words, to expose the targetarea 5-9 overlapping with sensor pattern 2, the main control unit 51 hasto select the tap D5 of the delay line 117 and set the delay pixel clockarea corresponding to the target area (or the fifth to ninth delayedpixel clock areas after the output of sensor pattern S1).

[0132] Performing such a sensing operation on the light beams b, c, dmakes it clear how each light beam is related in position to the outputof sensor pattern S1 by the first light beam. In actual printingoperation, the main control unit 51 selects the delayed clocks for thelight beams a to d and sets the printing area, using the positionalrelationship as correction data. This enables the main control unit 51to cause the image formation area for each beam to coincide with eachother with an accuracy of one-tenth of a clock (one-tenth of a pixel).

[0133] The main control unit 51 stores the obtained information on thelight beams a to d (information on the printing area and selection ofdelayed clock for each beam with respect to the first beam) in thememory 52. Storing the information in the memory 52 enables theapparatus to return to the original state as soon as the power supply isturned on again even after the power supply of the apparatus has beenturned off.

[0134] Furthermore, even when a new printing area is set in the mainscanning direction, storing the information in the memory 52 has theadvantage of needing only small adjustments and therefore eliminatingthe necessity of using unnecessary time for control.

[0135] Next, the operation of the main-scanning light-beam positionsensing circuit 40 a in the light beam sensor output processing circuit40 of FIG. 6 will be explained.

[0136] As explained earlier, the main control unit 51 can sense thelight beam position in the main scanning direction by changing theselection of the delayed clocks for the light beams a to d or theprinting area and monitoring the output of sensor pattern S2.Explanation will be given as to how the output of sensor pattern S2 istaken in by the main control unit 51.

[0137]FIG. 10 is a timing chart when the light beams a to d have notexposed sensor pattern S2 at all. As explained earlier, because thesample timer 120 forces the laser oscillators 31 a to 31 d to emitlight, sensor pattern S1 outputs a pulse signal once in a scanningoperation as a result of the first light beam exposing sensor patternS1.

[0138] The first counter 111 is a counter for counting the pulse signalfrom sensor pattern S1. For example, it counts from 0 to 7 endlessly andoutputs a carry signal at the end of the count of 7 as shown in thefigure. The second counter is a counter for counting the output ofsensor pattern S2.

[0139] The second counter 112 is cleared (reset) by the signal obtainedby delaying the carry signal from the first counter 111. The count ofthe second counter 112 becomes 0 at intervals of eight scans.

[0140] The latch circuit 113 latches the output value of the secondcounter 112. The latch timing of the latch circuit 113 is the leadingedge of the carry signal from the first counter 111. This enables thelatch circuit 113 to hold the value before the second counter 112 isreset.

[0141] The value held in the latch circuit 113 is updated when the firstcounter 111 outputs the next carry signal. As a result, the latchcircuit 113 always holds the count of the second counter 112 immediatelybefore the arrival of the carry signal.

[0142] In FIG. 10, because sensor pattern S2 has not sensed a light beamat all, the count of the second counter 112 is always 0 and thereforethe value held in the latch circuit 113 is also 0. As a result, the maincontrol unit 51 can tell from the latched value of 0 that sensor patternS2 has not sensed a light beam.

[0143]FIG. 11 is a timing chart to help explain the operation whensensor pattern S2 is constantly sensing a light beam. As shown in FIG.11, the second counter 112 for counting the output of sensor pattern S2counts from 0 to 8 in response to the output of sensor pattern S2.

[0144] The operation will be explained briefly. As soon as the countercarry output delay signal has cleared (reset) the second counter 112,the output of sensor pattern S2 is inputted to the counter 112, whichthen has the count of 1.

[0145] Thereafter, the output of sensor pattern S2 increments thecounter each time scanning is done. After scanning has been done eighttimes, the counter has the count of 8 and the value of 8 is held in thelatch circuit with the timing that the first counter 111 outputs a carrysignal. After the latch circuit 113 has held the value of 8, the secondcounter 112 is cleared (reset) to 0 and starts to count from 1.

[0146] While sensor pattern S2 is constantly sensing a light beam inthis way, the value held in the latch circuit 113 is 8. As a result,when the value in the latch circuit 113 is 8, the main control unit 51judges that sensor pattern S2 is constantly sensing a light beam.

[0147]FIG. 12 is a timing chart when sensor pattern S2 sometimes sensesa light beam and sometimes not or is in a delicate condition. Becausesensor pattern S2 sometimes senses a light and sometimes not, the countof the second counter 112 increases in a scanning operation and not inanother scanning operation. In the embodiment, because sensor pattern S2outputs a signal at intervals of two scan, the value held in the latchcircuit 113 is 4. As a result, reading the value of 4 held in the latchcircuit 113, the main control unit 51 judges that the edge of theprinting area is related delicately in position to sensor pattern S2.

[0148] Counting the output of sensor patterns S2 more than once in thisway has the following merits:

[0149] 1) The delicate positional relationship between the printing areaand sensor pattern S2 can be grasped.

[0150] 2) The main control unit 51 has only to read information atintervals of eight scans and bears a lower load than in reading atintervals of a scan.

[0151] It is desirable that the number of scans for each input of thelatched information (the period of the first counter) should be amultiple of the surfaces of a polygon mirror, taking into the accuracyof the surfaces of the polygon mirror. In the present embodiment, sincethe number of the surfaces of the polygon mirror is 8, the first counteris designed to output a carry signal at intervals of eight scans.

[0152] Next, the relationship between the light beam power and theprinting area setting accuracy will be explained.

[0153]FIG. 13 pictorially shows the exposure area in a case where thesame delayed clock is selected, the number of clocks in the printingarea is the same, and the light beam power is different. In FIG. 13, thelight beam power is the highest at A and decreases in this order: B andC.

[0154] As shown in the figure, the higher the light beam power, thelarger the area. As for the response of sensor pattern S2, it can beseen that the response of sensor pattern S2 differs even with the sameprinting area setting.

[0155] As shown in the figure, when the exposure edge of the light beamis in a position similar to that of the edge of sensor pattern S2,sensor pattern S2 makes a response or no response, depending on thelight beam power. In the example of FIG. 13, when the light beam poweris at A or B, the output a or b of sensor pattern S2 reaches a thresholdlevel of TH. The second counter 112 then counts the output. When thelight beam power is at C, the output c of sensor pattern S2 fails toreach the threshold level TH. As a result, the second counter 112 cannotcount the output.

[0156] Therefore, to align the printing areas of light beams with eachother with high accuracy, the power of each light beam must be madeequal before the printing areas are controlled.

[0157] Next, the light-beam passing (scanning) position control will beexplained in detail.

[0158]FIG. 14 is a diagram to help explain the light-beam passingposition control when the light beam sensing unit 38 of FIG. 3 is used.The portions related to the light-beam passing position control in theblock diagram of FIG. 4 have been extracted and shown in detail.

[0159] As explained earlier, the sensor patterns S1, S2 of the lightbeam sensing unit 38 output pulse signals indicating that light beamshave passed. The sensor patterns SA to SG output independent signalsaccording to the passing positions of light beams.

[0160] The output signals of the sensor patterns SA, SG are inputted toamplifiers 61, 62 (hereinafter, sometimes referred to as amplifiers A,G), respectively. The amplification factor of each of the amplifiers 61,62 is set by the main control unit 51 composed of a CPU.

[0161] As explained earlier, the galvanomirrors 33 a to 33 d arecontrolled so that the light beam may pass over sensor pattern AS or SG.Monitoring the output of sensor pattern SA or SG enables the relativelight beam power on the photosensitive drum 15 to be sensed.

[0162] The output signals of the sensor patterns SB to SF are inputtedto differential amplifiers 63 to 66 (hereinafter, sometimes referred toas differential amplifiers B-C, C-D, D-E, E-F) for amplifying thedifference between the adjacent output signals from the sensor patternsSB to SF, respectively. The differential amplifier 63 amplifies thedifference between the output signals from the sensor patterns SB, SC;the differential amplifier 64 amplifies the difference between theoutput signals from the sensor patterns SC, SD; the differentialamplifier 65 amplifies the difference between the output signals fromthe sensor patterns SD, SE; and the differential amplifier 66 amplifiesthe difference between the output signals from the sensor patterns SE,SF.

[0163] The output signals from the amplifiers 61 to 66 are inputted to aselect circuit (or an analog switch) 41. According to a sensor selectsignal from the main control unit (CPU) 51, the select circuit 41selects a signal to be outputted to an integrator 42. The output signalof the amplifier selected by the select circuit 41 is integrated at theintegrator 42.

[0164] The pulse signal from sensor pattern S1 is also inputted to theintegrator 42. The pulse signal from sensor pattern S1 is used as areset signal (or integration start signal) that resets the integrator 42and simultaneously starts a new integrating operation. The functions ofthe integrator 42 is to remove noise and eliminate the effect of theinclination with which the light beam sensing unit 38 has beeninstalled, which will be described in detail later.

[0165] The output of the integrator 42 is inputted to an A/D converter43. The pulse signal from sensor pattern S2 is also inputted to the A/Dconverter 43. When receiving the signal from sensor pattern S2 as aconversion start signal, the A/D converter 43 starts analog-to-digitalconversion. Namely, A/D conversion is started with the timing that alight beam passes over the sensor pattern S2.

[0166] As described above, immediately before the light beams pass overthe sensor patterns SA to SG, the pulse signal from sensor pattern S1resets the integrator 42 and at the same time, starts integration. Asresult, while the light beams are passing over the sensor patterns SA toSG, the integrator 42 integrates the signals indicating the passingpositions of the light beams.

[0167] Then, immediately after the light beams have passed over thesensor patterns SA to SG, the pulse signal from sensor pattern 2triggers the A/D converter 43 to A/D convert the result of integrationat the integrator 42. This enables the sense signal with less noise fromwhich the effect of the inclined installation of the light beam sensingunit 38 has been removed to be converted into a digital signal in lightbeam passing position sensing.

[0168] After the A/D conversion, the A/D converter 43 outputs aninterrupt signal INT indicating the completion of the process to themain control unit 51.

[0169] The amplifiers 61 to 66, select circuit 41, integrator 42, andA/D converter 43 constitute the light beam sensor output processingcircuit 40.

[0170] In this way, the light beam power sensing signal and light beamposition sensing signal converted into digital signals are inputted tothe main control unit 51 as relative light beam power information andlight beam position information on the photosensitive drum 15. The maincontrol unit 51 determines the power and passing position of each lightbeam on the photosensitive drum 15.

[0171] On the basis of the relative light beam power sensing signal andlight beam position sensing signal on the photosensitive drum 15, themain control unit 51 sets the light-emitting power for each of the laseroscillators 31 a to 31 d and calculates the controlled variable for eachof the galvanomirrors 33 a to 33 d. The results of calculation arestored in the memory 52, as the need arises. The main control unit 51sends the results of calculation to the laser drivers 32 a to 32 d andthe galvanomirror driving circuits 39 a to 39 d.

[0172] As shown in FIG. 6, the galvanomirror driving circuits 39 a to 39d include latches 44 a to 44 d for storing the results of calculation,respectively. Once the main control unit 51 has written the data intothe latches, the values remain unchanged until the data is updated.

[0173] The data items held in the latches 44 a to 44 d are converted byD/A converters 45 a to 45 d into analog signals (or voltages), which arethen inputted to drivers 46 a to 46 d for driving the galvanomirrors 33a to 33 d. The drivers 46 a to 46 d drive the galvanomirrors 33 a to 33d according to the analog signals from the D/A converters 45 a to 45 d.

[0174] In the embodiment, because only one of the amplified outputsignal of the sensor patterns SA to SG is selected by the select circuit41, integrated, and A/D converted, the output signals of the sensorpatterns SA to SG cannot be inputted to the main control unit 51 at atime.

[0175] Therefore, to sense the power of the light beam, it is necessaryto move the passing position of the light beam to over sensor pattern SAor SG and switch the select circuit 41 so that the output signal fromthe corresponding sensor pattern may be inputted to the main controlunit 51.

[0176] When the passage of a light beam is unknown, it is necessary todetermine the passing position of the light beam by switching the selectcircuit 41 sequentially and inputting the output signals from all thesensor patterns SA to SG to the main control unit 51 one after another.

[0177] Once where the light beam is passing has been determined, theposition at which the light beam will pass can be estimated, so theoutput signals of all the sensor patterns do not always have to beinputted to the main control unit 51.

[0178] Next, the operation of the printer unit 2 at the time of thepower supply being turned on will be described by reference to aflowchart shown in FIG. 15. Explanation of the operation of the scannerunit 1 will be omitted.

[0179] When the power supply for the copying machine is turned on, themain control unit 51 rotates the fixing rollers in the fixing unit 26and starts heating control of the fixing unit 26 (S311, S312). Then, alight beam power control routine is executed, which controls the powerof each light beam on the photosensitive drum 15 so that the power maybe equal for the respective beams (S313).

[0180] After the power of each light beam on the photosensitive drum 15has been controlled so as to have the same level, an offset correctionroutine is executed, which senses the offset value of the light beamsensor output processing circuit 40 and performs a correction processfor the offset value (S314). Then, a light beam passing position controlroutine is executed (S315).

[0181] Next, a main-scanning light-beam position control routine isexecuted (S316). Then, the photosensitive drum 15 is rotated andprocess-related initialization, including the maintenance of the surfaceconditions for the photosensitive drum 15, is executed (S317).

[0182] After a series of such initialization actions, the fixing rollersare rotated until the temperature of the fixing unit 26 has risen to aspecific temperature and the copying machine goes into the wait state(S318). When the temperature of the fixing unit 26 has risen to thespecific temperature, the rotation of the fixing rollers is stopped(S319) and the copying machine goes into the copy instruction wait state(S320).

[0183] In the copy instruction wait state (S320), when no copying (orprinting) instruction has been received from the control panel 53, if,for example, 30 minutes have elapsed since the preceding light beampassing position control routine was executed (S321), a light beam powercontrol routine will be executed automatically (S322) and then an offsetcorrection routine will be executed automatically (S323). Thereafter,the light beam passing position control routine and main-scanninglight-beam position control routine will be executed again (S324, S325).After this, control will return to step S320, where the copying machinewill go into the copy instruction wait state again.

[0184] In the copy instruction wait state (S320), when a copyinstruction is received from the control panel 53, a check is made tosee if there is an instruction to change the resolution (S326). As aresult of the check, if there is an instruction to change theresolution, the number of revolutions of the polygon motor 36 is changedto a value suited for the specified resolution.

[0185] Then, a suitable one for the resolution is selected from thecrystal oscillators 114 a to 114 d (S328). The light-beam power controlroutine is then executed (S329). Thereafter, the offset correctionroutine is executed (S330). Then, the light-beam passing positioncontrol routine is executed (S331). After this, the main-scanninglight-beam position control routine is executed (S332) and the copyoperation is performed (S333).

[0186] If the result of the check at step S326 has shown that there isno instruction to change the resolution, the light-beam power controlroutine will be executed (S329) because neither the number ofrevolutions of the polygon motor 36 nor the crystal oscillator will bechanged. Then, the offset correction routine will be executed (S330).Thereafter, the light-beam passing position control routine (S331), themain-scanning light-beam position control routine (S332), and the copyoperation (S333) will be executed in that order.

[0187] After the copy operation has been completed, control returns tostep S320 and the copying machine repeats the above operations.

[0188] In this way, even in the intervals between one copy operation andanother, the light-beam power control routine, light-beam passingposition control routine, and main-scanning light-beam position controlroutine are executed. This enables an image to be formed in the bestcondition constantly even when a lot of copies are made consecutively.

[0189] A first example of the light beam power control routine at stepsS313, S322, and S329 in FIG. 15 will be described by reference to theflowcharts shown in FIGS. 17 and 18.

[0190] The main control unit 51 sets the amplification factor of theamplifier 61(A) at a specific value (S231). With each light beam passingover the sensor pattern SA, when the output of the amplifier 61(A) isintegrated at the integrator 42 and A/D converted at the A/D converter43, use of the specific value prevents the resulting value from beingsaturated and allows the value to change in proportion to the power ofthe light beam.

[0191] Next, the main control unit 51 turns on the polygon motor 36,thereby rotating the polygon mirror 35 at a specific number ofrevolutions (S232). Then, the main control unit 51 forces the laseroscillator 31 a to emit light at a specific value stored in the memory52 (S233). After this, the polygon mirror 35 causes the light beam a tostart scanning. Here, the specific value is a value suitable for imageformation at that time.

[0192] In general, in an image forming apparatus using electronicphotographic processing, the power of the light beam must be changed,depending on the environment in which the image forming apparatus isinstalled or on its using conditions (including aging). The memory 52stores data on the appropriate power of each light beam under suchvarious conditions.

[0193] Then, the main control unit 51 controls the galvanomirror 33 a sothat the light beam a may pass over sensor pattern SA (S234). The lightbeam a has to pass through almost the center of sensor pattern SA so asnot to stray from sensor pattern SA. If the beam a deviated from sensorpattern SA, the sensed power would have a smaller value.

[0194] Since the sensor pattern SA (or SG) used for light beam powercontrol has a sufficient size (with a length of about 900 μm in thesub-scanning direction) as described in FIG. 3, it is impossible thatsuch a problem will take place.

[0195] Then, when the light beam s starts to pass over the sensorpattern SA, the A/D converter 43 inputs a value proportional to thepower of the beam a to the main control unit 51. The main control unit51 stores the value (preferably, the average value of an integralmultiple of the number of faces of the polygon mirror 35) into thememory 52 as the optical power Pa of the light beam a on thephotosensitive drum 15 (S235) and turns off the laser oscillator 31 a(S236).

[0196] Next, the main control unit 51 forces the laser oscillator 31 bto emit light (S237) and controls the galvanomirror 33 b as with thelaser beam a, thereby causing the light beam b to pass over the sensorpattern SA (S238).

[0197] Then, the A/D converter 43 inputs a value proportional to thepower of the beam b on the photosensitive drum 15 to the main controlunit 51. The main control unit 51 determines the value to be the opticalpower Pb and compares it with the optical power Pa of the light beam aon the photosensitive drum 15 stored in the memory 52 (S239). In thecase of the light beam b, too, it is desirable that the output value ofthe A/D converter 43 should be taken in as many times as an integralmultiple of the number of faces of the polygon mirror 35 and the averageof the output values be determined to be Pb.

[0198] As a result of comparing the optical power Pa of the beam a withthe optical power Pb of the beam b on the photosensitive drum 15, if thedifference is smaller than or equal to a specific value (ΔP)(preferably, “1”), there will be no problem in terms of picture quality.If the difference is larger than the value, a picture quality problemwill arise and correction be needed.

[0199] For example, as a result of comparing the optical power Pb withthe optical power Pa, if Pb is larger than Pa and the difference betweenthem is larger than ΔP (S240, S241), decreasing the light-emitting powerset value for the laser driver 32 b will enable the optical power of thelight beam b on the photosensitive drum 15 to be decreased (S242).

[0200] Conversely, as a result of comparing the optical power Pb withthe optical power Pa, if Pa is larger than Pb and the difference betweenthem is larger than ΔP (S240, S241), increasing the light-emitting powerset value for the laser driver 32 b will enable the optical power of thelight beam b on the photosensitive drum 15 to be increased (S243).

[0201] After having corrected the optical power of the beam b on thephotosensitive drum 15, the main control unit 51 stores thelight-emitting power set value at that time into the memory 52 as thevalue for the laser oscillator 31 b (S244). Then, it returns control tostep S239, senses the optical power of the beam b on the photosensitivedrum 15 again, compares Pb with Pa, and repeats correction until thedifference between them becomes equal to or smaller than ΔP.

[0202] In this way, the difference between the power of the beam a andthat of the beam b can be made equal to or smaller than the specificvalue (ΔP).

[0203] The light beams c and d are processed in a similar manner atsteps S245 to S264, thereby enabling the difference in optical powerbetween the light beams a, b, c, and d on the photosensitive drum 15 tobe equal to or smaller than the specific value (ΔP).

[0204] While in the example, the light beam a has been used as areference, the light beam b, c, or d may be used as a reference. It isdesirable that the specific value (ΔP) should be made 1% or less of thereference (the value of Pa).

[0205] The main-scanning light-beam position control routine at stepsS316, S325, and S332 in FIG. 15 will be described by reference to theflowchart shown in FIG. 19.

[0206] The main control section 51 first acquires main-scanninglight-beam position information on the light beam a (S341). Here, themain-scanning light-beam position information means that such set valuesin the image transfer clock generating sections (printing area settingsections) 119 a to 119 d and such selected information as makes patternS2 of the light beam sensing unit 38 become the edge of the exposurearea (printing area). A method of acquiring the information will beexplained later in detail.

[0207] Similarly, main-scanning light-beam position information on lightbeam b, light beam c, and light beam d is also acquired (S342 to S344).

[0208] After having acquired main-scanning light-beam positioninformation on the light beams a to d, the main control unit 51 sets thenecessary printing area for actual image formation (copying or printing)(S345). The necessary printing area for actual image formation (copyingor printing) is set according to main-scanning light-beam positioninformation on the light beams a to d, the size of sheets used for imageformation (copying or printing), and the filing margin.

[0209] For example, it is assumed that data items as shown in Table 1have been acquired as main-scanning light-beam position information onthe light beams a to d. TABLE 1 PRINTING AREA START & END DELAYED CLOCKBEAM a 5 to 9 D5 BEAM b 12 to 16 D8 BEAM c 18 to 22 D2 BEAM d 20 to 24D7

[0210] If the size of sheets used for actual image formation is, forexample, A4 lateral with no settings, including a filing margin, theprinting area will contain 7015 (≈297×600÷25.4) pixels with a resolutionof 600 dpi.

[0211] Here, it is assumed that the distance between sensor pattern S2of the light beam sensing unit 38 and the left end of the actualprinting area is equivalent to 100 pixels. In this case, the pixel clockarea for each light beam corresponding to the printing area (or targetimage formation area) is set as shown in Table 2. TABLE 2 PRINTING AREASTART & END DELAYED CLOCK BEAM a 109 to 7124 D5 BEAM b 116 to 7131 D8BEAM c 122 to 7137 D2 BEAM d 124 to 7139 D7

[0212] After the stetting has been done, when delayed clock D5 isselected for light beam a, D8 is selected for light beam b, D2 isselected for light beam c, and D7 is selected for light beam d, theprinting areas of the light beams a to d coincide with each other withan accuracy of one-tenth of a pixel.

[0213]FIGS. 20, 21, and 22 are flowcharts to help explain routines foracquiring main-scanning light-beam information on light beam a at stepS341 in FIG. 19. Explanation of the beam a applies to the beams b to d.

[0214] In preparation for acquiring information on light beam a, themain control unit 51 sets, for example, the print start of 7400 and theprint end of 7401 in the image transfer clock generating sections(printing area setting sections) 119 b to 119 d (S351). In step S351,the printing (exposure) areas other than that of light beam a are movedfurther away from the light beam sensing unit 38 and the photosensitivedrum 15 is also moved to a place where it will not be exposed. Step 351is the necessary step to prevent the light beams a to d from interferingwith each other. Step S351 makes it possible to acquire accurateinformation only on light beam a.

[0215] Next, the main control unit 51 sets the delayed clock selector118 a so that delayed clock D1 may be used for image formation (S352).Then, the main control unit 51 sets a print start position of 1 and aprint end position of 5 in the image transfer clock generating section(printing area setting section) 119 a (S353).

[0216] Thereafter, the main control section 51 sets paint black testdata in the image data I/F 56, causes the laser oscillator 31 a to emitlight according to the paint black test data, stops resetting the firstcounter 111, and causes the beam to scan the scanning surface a specificnumber of times, or eight times, according to the number of the surfacesof the polygon mirror (S354). As explained earlier, the operation causesthe printing area 1-5 (an area equivalent to five dots) to be exposedwith sensor pattern S1 of the light beam sensing unit 38 as a reference.

[0217] Then, the main control unit 51 takes in the data held in thelatch circuit 113 in response to the interrupt signal (carry) from thefirst counter 111, resets the first counter (S355), and judges whetherthe value is 8 (check 1: S356).

[0218] When the result of the judgment has shown that the taken-in datais not 8, this means that sensor pattern S2 of the light beam sensingunit 38 has not been exposed sufficiently. Therefore, after adding avalue of 1 to each of the set values (print start and end) in the imagetransfer clock generating section (printing area setting section) 119 aand shifting the printing area (exposure area) one pixel, the maincontrol unit 51 stops resetting the first counter 111 (S357), and waitsfor the interrupt signal from the first counter 111.

[0219] When the result of the judgment at step S356 has shown that thetaken-in data is 8, this means that sensor pattern S2 of the light beamsensing unit 38 has been exposed sufficiently. In this case, afterfurther adding a value of 1 to each of the set values (print start andend) in the image transfer clock generating section (printing areasetting section) 119 a and shifting the printing area (exposure area)one pixel, the main control unit 51 stops resetting the first counter111 and causes the beam to scan the scanning surface a specific numberof times or eight times (S358).

[0220] Next, the main control unit 51 takes in the data held in thelatch circuit 113 in response to the interrupt signal (carry) from thefirst counter 111 (S359) and judges whether the value is 8 (check 2:S360).

[0221] When the result of the judgment has shown that the taken-in datais not 8, the main control unit 51 adds a value of 1 to each of the setvalues (print start and end) in the image transfer clock generatingsection (printing area setting section) 119 a as in the precedingprocess, shifts the printing area (exposure area) one pixel, stopsresetting the first counter 111 (S357), and makes the first check (check1) again.

[0222] When the result of the judgment at step S360 has shown that thetaken-in data is 8, the values obtained by subtracting 2 from the setvalues (print start and end) in the image transfer clock generatingsection (printing area setting section) 119 a are set in the imagetransfer clock generating section (printing area setting section) 119 a(S361). The values at this time are stored in the memory 52 asmain-scanning light-beam position information on light beam a in unitsof one pixel (S362).

[0223] By those operations, the main control section 51 can shift the5-pixel printing area (exposure area) one pixel by one pixel, recognizehow many pixels it has to shift the printing area before the printingarea reaches sensor pattern S2 of the light beam sensing unit 38, andstores the value immediately before the arrival at sensor pattern S2into the memory 52.

[0224] In the embodiment, although the printing area (exposure area) hasreached sensor pattern S2 of the light beam sensing unit 38 (althoughthe check 1 has been made), it is verified that the printing area(exposure area) has been further shifted one pixel and sensor pattern S2has been exposed (or that the check 2 has been made). The reason is thateven when the sensor pattern S2 of the light beam sensing unit 38 hasresponded to unnecessary stray light that might develop in the opticalsystem unit, the main control unit 41 has to distinguish from thecorrect response from the unwanted response.

[0225] Generally, the energy of stray light is much lower than that ofthe main light beam and sensor pattern S2 of the light beam sensing unit38 will hardly respond to such stray light. For some reason, however,sensor pattern S2 could respond. To avoid such erroneous responses, itis necessary to make sure that sensor pattern S2 responses reliably (orsenses light reliably) even when the printing area (exposure area) hasbeen shifted extra several pixels since sensor pattern S2 began toresponse.

[0226] In the embodiment, to simplify explanation, the printing area(exposure area) has been made up of five pixels and the amount of shiftto make the above checks has been determined to be one pixel. Thepresent invention is not restricted to these values. The size of theprinting area (exposure area) and the number of checks may be determinedsuitably, taking into account the size of sensor pattern S2 of the lightbeam sensing unit 38.

[0227] After main-scanning light-beam position information on light beama has been acquired in units of one pixel, the main control unit 51carries out the operation of acquiring main-scanning light-beam positioninformation in units of one-tenth of a pixel.

[0228] With delayed clock D1 being selected, the printing area (exposurearea) has been set in units of one pixel and is in a positionimmediately before sensor pattern S2 of the light-beam sensing unit 38starts to respond. In this state, the main control unit 51 switches thedelayed clock from D1 to D2 (or shifts the position of the clockone-tenth of a clock), stops resetting the first counter 111, causes thebeam to scan the scanning surface eight times, and waits for aninterrupt signal (S363).

[0229] Receiving the interrupt signal from the first counter 111, themain control unit 51 takes in the value from the latch circuit 113 ofthe main-scanning beam position sensing circuit in the light-beamposition sensing output circuit 40 and resets the first counter 111(S364).

[0230] Next, the main control unit 51 judges whether the value in thelatch circuit 113 is 8 (S365) and checks whether the printing area(exposure area) has reached sensor pattern S2 of the light-beam sensingunit 38.

[0231] When the result of the check has shown that the latched value isnot 8, this means that the printing area (exposure area) has not reachedsensor pattern S2 yet. The main control unit 51 returns control to stepS363, selects a {fraction (1/10)}-clock-shifted delayed clock, and makesjudgments as described above.

[0232] When the result of the check at step S365 has shown that thelatched value is 8, this means that the printing area (exposure area)has reached sensor pattern S2. Namely, because the printing area hasreached the target area, the main control unit 51 uses the delayed clockin image formation (e.g., copying or printing) and stores the selecteddelayed clock in the memory 52 (S366).

[0233] As described above, the main control unit 51 can move theprinting area (exposure area) in units of about one-tenth of a pixel onthe basis of the setting of the printing area (exposure area) and theselection of the delayed clock. Checking the response from sensorpattern S2 of the light beam sensing unit 38 at that time, the maincontrol unit 51 acquires main-scanning light-beam position informationon light beam a with an accuracy of about one-tenth of a pixel.

[0234] In the embodiment, the printing area (exposure area) has beenshifted one by one to the downstream side in the direction of light beamscanning to find a point at which sensor pattern S2 of the light beamsensing unit 38 responds. The present invention is not limited to this.

[0235] For instance, the printing area (exposure area) may be furthershifted one by one to the downstream side in the direction of light beamscanning to find a point at which sensor pattern S2 of the light beamsensing unit 38 makes no response. Then, the point may be used asmain-scanning light-beam position information on each of the light beamsa to d.

[0236] Furthermore, the printing area (exposure area) may have been seton the downstream side in the direction of light beam scanning and beshifted one by one to the upstream side to find a point where sensorpattern S2 of the light beam sensing unit 38 makes a response or noresponse.

[0237] As explained in detail, according to the present invention, thereare provided a light beam scanning apparatus and an image formingapparatus which enable the relative exposure position to be alwayscontrolled accurately even when the relationship between the mainscanning positions of light beams is unknown.

[0238] Furthermore, according to the present invention, there isprovided a beam scanning apparatus applicable to an image formingapparatus with more than one resolution.

[0239] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

1. A light beam scanning apparatus comprising: a plurality of beamgenerating means for generating light beams; scanning means foroptically combining the light beams generated at said beam generatingmeans, reflecting the combined beams to a scanning surface including thesurface of an image retaining element, and causing said light beams toscan said scanning surface; first sensing means, provided near saidimage retaining element, for sensing the first one of said light beamscaused by said scanning means to scan; clock generating means forgenerating a pixel clock to be used in said beam generating means foreach of said light beams, in response to a sense signal indicating thatsaid first beam has exposed said first sensing means; second sensingmeans, provided on the more downstream side in the main scanningdirection than said first sensing means and, for sensing said lightbeams; first control means for giving control data to said clockgenerating means so that said light beams may expose a target areabetween said first and second sensing means; and image formation areasetting means for determining a pixel clock area corresponding to atarget image formation area on said image retaining element on the basisof the control data from said control means and setting the pixel clockarea in said clock generating means.
 2. A light beam scanning apparatusaccording to claim 1 , wherein said clock generating means includes:clock means for generating a clock signal a specific time after saidfirst beam exposed said first sensing means; delay means for delayingsaid clock signal generated at said clock means, selecting the delay ofsaid clock in a range of one clock or less for each beam, and providinga delayed clock signal as a pixel clock to be used to generate a beam;and clock setting means for setting, for the respective beams, exposurepixel clock areas used by said beam generating means in the pixel clocksgiven by said delay means and providing exposure pixel clocks.
 3. Alight beam scanning apparatus according to claim 2 , wherein said delaymeans includes a delay line and delay clock selectors, said delay linehaving taps, and each of said delay clock selectors designed to selectand output a delayed clock generated at one tap, which is to be used togenerate one beam.
 4. A light beam scanning apparatus according to claim3 , wherein the target area between said first and second sensing meansis an area overlapping with said second sensing means, said light beamscanning apparatus further comprising: a counter for counting the numberof times said beams expose said second sensing means; second controlmeans for driving one of said beam generating means using the exposurepixel clock obtained from said clock setting means controlled on thebasis of said control data and causing said scanning means to scan saidscanning surface a specific number of times; comparison means forreading a value in said counter after said second control means hasscanned the specific number of times and comparing the value with apredetermined number; means for selecting the next tap whose amount ofdelay is greater than that of said selected tap when the result of thecomparison at said comparison means has shown that the value in saidcounter is smaller than said predetermined number; and amount-of-delaysetting means, when the result of the comparison at said comparisonmeans has shown that the value in said counter is equal to saidpredetermined number, for setting a delay selected at that time as theamount of delay for the one of said beam generating means.
 5. A lightbeam scanning apparatus according to claim 1 , wherein the operationscarried out by said first control means and said image formation areasetting means are executed immediately after the power supply of saidlight beam scanning apparatus has been tuned on.
 6. A light beamscanning apparatus according to claim 1 , wherein the operations carriedout by said first control means and said image formation area settingmeans are executed at specific intervals of time.
 7. A light beamscanning apparatus according to claim 1 , further comprising drumexposure inhibit means for controlling said beam generating means insuch a manner that said light beams caused by said scanning means toscan are prevented from exposing said image retaining element, whereinthe operations carried out by said first control means and said imageformation area setting means are executed after an operation of saiddrum exposure inhibit means has been started.
 8. A light beam scanningapparatus according to claim 1 , further comprising resolution settingmeans for setting image resolution at which an image is retained on saidimage retaining element; scanning speed changing means for changing thescanning speed of said scanning means according to the resolution set bysaid resolution setting means; and clock changing means for changing thefrequency of a clock generated by said clock generating means accordingto the resolution set by said resolution setting means.
 9. A light beamscanning apparatus according to claim 1 , further comprising light beampower sensing means for sensing the power of each of said light beamscaused by said scanning means to scan said scanning surface; and lightbeam power control means for controlling said light beam generatingmeans on the basis of the result of sensing at said light beam powersensing means in such a manner that the power of each of said lightbeams lies in a specific range.
 10. An image forming apparatuscomprising: image reading means for optically reading the image of adocument and providing image data corresponding to the image of thedocument; a plurality of beam generating means for generating lightbeams corresponding to said image data provided by said reading means;scanning means for optically combining the light beams generated at saidbeam generating means, reflecting the combined beams to a scanningsurface including the surface of a photosensitive drum and causing saidlight beams to scan said scanning surface; first sensing means, providednear said photosensitive drum, for sensing the first one of said lightbeams caused by said scanning means to scan; clock generating means forgenerating a pixel clock to be used in said beam generating means foreach of said light beams, in response to a sense signal indicating thatsaid first beam has exposed said first sensing means; second sensingmeans, provided on the more downstream side in the main scanningdirection than said first sensing means, for sensing said light beams;first control means for giving control data to said clock generatingmeans so that said light beams may expose a target area between saidfirst and second sensing means; setting means for determining a pixelclock area corresponding to a target image formation area on saidphotosensitive drum on the basis of the control data for said controlmeans and setting the pixel clock area in said clock generating means;formation means for forming, by means of said beam generating means, anelectrostatic latent image corresponding to the image data provided bysaid reading means in an image formation area on said photosensitivedrum corresponding to said pixel clock area set by said setting means;and printing means for printing on a sheet of paper a visible imagecorresponding to the electrostatic latent image formed on saidphotosensitive drum.
 11. An image forming apparatus according to claim10 , wherein said clock generating means includes: clock means forgenerating a clock signal a specific time after said first beam exposedsaid first sensing means; delay means which delays said clock signalgenerated at said clock means, selects the delay of said clock in arange of one clock or less for each beam, and provides a delayed clocksignal as a pixel clock to be used to generate a beam; and clockselecting means for selecting, for the respective beams, exposure pixelclock areas used by said beam generating means in the pixel clocks givenby said delay means and providing exposure pixel clocks.
 12. A lightbeam scanning apparatus according to claim 11 , wherein said delay meansincludes a delay line and delay clock selectors, said delay line havingtaps, and each of said delay clock selectors designed to select andoutput a delayed clock generated at one tap, which is to be used togenerate one beam, and said clock selecting means includes a pluralityof pixel clock setting means for setting the exposure pixel clock areacorresponding to said target area in the pixel clocks provided from saiddelayed clock selector for each of the beams on the basis of saidcontrol data.
 13. An image forming apparatus according to claim 10 ,further comprising: resolution setting means for setting imageresolution at which an image is formed on said photosensitive drum;scanning speed changing means for changing the scanning speed of saidscanning means according to the resolution set by said resolutionsetting means; and clock changing means for changing the frequency of aclock generated by said clock generating means according to theresolution set by said resolution setting means.
 14. A light beamscanning method comprising: the step of causing light beams generated bylight beam generators to scan a scanning surface including the surfaceof a photosensitive drum; the step of causing a first sensor providednear said photosensitive drum to sense the first one of said lightbeams; the step of causing a second sensor provided on the moredownstream side in the main scanning direction than said first sensor tosense said light beams; the step of giving control data to an pixeltransfer clock generator so that said light beams may expose a targetarea between said first and second sensors and thereby generating anexposure pixel clock for each of the light beams; and the step ofdetermining a pixel clock area corresponding to an image formation areaon said photosensitive drum on the basis of said control data andsetting the pixel clock area in said clock generator.
 15. A light beamscanning method according to claim 14 , wherein said clock generatingstep includes: the step of generating a clock a specific time after saidfirst beam exposed said first sensing means; the step which delays saidgenerated clock, selects the delay of said clock in a range of one clockor less for each beam, and provides a delayed clock as said pixel clockfor each beam; and the step of selecting an exposure clock areacorresponding to said target area in said pixel clocks and providingexposure pixel clock for each of said beams.
 16. A light beam scanningmethod according to claim 15 , wherein said delaying step includes adelay selecting step of selecting, for each of said beams, a delayedclock generated at one of the taps of a delay line for delaying saidclock on the basis of said control data and outputting the selectedclock as said pixel clock, and said exposure pixel clock area selectingstep includes the step of setting the exposure pixel clock areacorresponding to said target area in the pixel clocks provided in saiddelay selecting step for each of the beams on the basis of said controldata.
 17. A light beam scanning method according to claim 16 , whereinthe target area between said first and second sensors is an areaoverlapping with said second sensor, said light beam scanning methodfurther comprising: the step of driving one of said beam generatorsusing said exposure pixel clock generated on the basis of said controldata and scanning said scanning surface a specific number of time; thestep of counting the number of times said beams expose said secondsensor; the step of reading the counted number of exposures afterscanning has been done said specific number of times and comparing itsvalue with a predetermined number; the step of selecting the next tapwhose amount of delay is greater than that of said selected tap when theresult of said comparison has shown that said number of exposures issmaller than said predetermined number; and the step of setting, whenthe result of said comparison has shown that said number of exposures isequal to said predetermined number, the tap selected at that time as theamount of delay for the one of said beam generating means.